A cerebral aneurysm clip (clamp) is a surgical instrument for clamping the base part of a cerebral aneurysm to either temporarily or permanently isolate the latter from the cerebral artery. For this purpose, the aneurysm clip must be able to maintain the clamping pressure with high reliability as long as desired without causing injury to the wall of the blood vessel to which it is attached. Such injury could be caused by shearing action resulting from improper alignment of the jaws of the clip, or by improper magnitude of pressure, or by introduction of foreign material trapped in the cracks and crevices of a poorly constructed clip, or by the electromagnetic properties of clips made of improperly chosen materials used in their construction. There are a number of different kinds of cerebral aneurysm clips available in the art today; however, these are made of improperly chosen materials which interfere with important diagnostic techniques such as magnetic resonance imaging (MRI or NMR) due to haloing caused by the magnetic materials.
In order to provide satisfactory and prolonged service when properly implanted, a cerebral aneurysm clip must satisfy the following criteria;
1. The clamping pressure of the jaws of the clip must be sufficient for isolating a cerebral aneurysm, but not so great that it could damage the wall of the blood vessel to which it is attached whether used in a permanent implant or in a temporary implant application mode.
2. The clamping pressure of the clip must not change with time to prevent the clip from being displaced of slipping off.
3. Since aneurysm clips are often left permanently in the brain, the size of the clip must be as small as possible and it should not be made of ferromagnetic materials or materials which cause haloing in magnetic resonance imaging.
4. The shape of the clip should be such that the surgeon who is using it in an implant can view it clearly at all times without his view being obstructed during the implant operation.
5. To prevent unintentional tears and pressure points, jagged edges must be absent from the exterior surface of the clip.
6. Cracks in the exterior surface of the clip, even extremely fine micro-cracks, crevices and other similar defects should be absent from the surface of the clip in order to reduce the possibility of trapping foreign matter and contamination.
7. In order to prevent scissoring of the vessel wall to which the clip is attached, it is imperative that the jaws of the clip be properly aligned at all times both in its opened and closed state.
8. It is highly desirable, if not imperative, for the surgeon implanting the clip to be able to easily distinguish between seemingly identical clips sizewise which have different ranges of closing force, a requirement largely used to distinguish between temporary and permanent implants.
A basic aneurysm clip having no provision for maintaining jaw alignment, such as is described in U.S. Pat. No. 3,827,438, issued Aug. 6, 1974, or U.S. Pat. No. 4,024,868, issued May 24, 1977, would not be acceptable for implant purposes under the above-noted criteria.
A number of aneurysm clips having differently designed jaw guides for keeping the clamping jaws properly aligned have been proposed and are available. The simplest aneurysm cllips with provision for maintaining jaw alignment resemble electrical clamps where a window is provided in one arm to confine the movement of the second arm. Such a clamp is described in U.S. Pat. No. 4,484,581 issued Nov. 27, 1984. Aneurysm clips of this type usually are made of rather wide strips of material which unfortunately obstruct the view of the surgeon implanting the device. Furthermore, the width of the jaws prevents precision clamping of the narrow aneurysm base. Where the dimensions of this type of known clip are reduced to dimensions more typical in cerebral implants (a few millimeters to 1-2 centimeters), this known type of clip must be made of stainless steel to provide adequate strength in the "window frame" and in the thin arm portion which glides inside the frame. The variation of this type of "window frame" alignment is a partial window frame enclosed by a wide bridging member welded in place over the partial window as described in the above-noted U.S. Pat. No. 4,484,581.
U.S. Pat. No. 4,192,315, issued Mar. 11, 1980 discloses an aneurysm clip in which the clamping ends of the clip are supported by cross arms which cross inside the annulus of a ring loop. The disadvantage of this design is the presence of the protruding, loose loop, which if tightened would interfere with the smooth movement of the clamping jaws. U.S. Pat. No. 3,598,125 is another example of a clip having too much bulk and exterior protuberants for safe implantation in humans.
Another known aneurysm clip is described in U.S. Pat. No. 4,360.023, issued Nov. 23, 1982. This known clip uses a metal guide wire consisting of two leg portions and a straight guide portion which bridges the crossing region of the cross arms that support the clamping jaws of the clip. The disadvantage of this construction becomes immediately apparent when the clips are viewed at 50 times or higher magnification. Holes are drilled through the two arm portions in which the leg portions of the guide wire are mounted and then the leg portions are riveted into the holes for securely imbedding and fixing the straight guide portions into bridging relationship over the crossing arm portions. Unacceptable microcracks, voids and crevices are produced and are clearly visible on both sides of the two cross arms. Furthermore, micro-cracks are formed and clearly visible on the two clamping arm portions of the clip. The clips which have been examined were made out of chrome-cobalt alloy steel. Defects such as those mentioned above would be even more pronounced and dangerous if titanium or titanium alloys were used to fabricate the design, because titanium is prone to embrittlement and micro-cracking. At the same time, titanium and some titanium alloys (e.g. 6-4 alloys) are most desirable for implants since they are inert, compatible with body fluids, are non-magnetic and do not produce MRI haloes.
Currently, some of the known aneurysm clips are color coded. Since the temporary clips generally have closing forces in the 40-80 grams range, while the clips to be permanently implanted have closing forces in the 110-150 grams range, the temporary clips often are color coded for easy identification. This coloring customarily is done by the application of yellow electroplated gold to the clamping arms portion. However, any electroplating operation presents an inherent danger of peeling due to poor adhesion. This well known fact is aggravated by the further fact that chrome-cobalt alloys and similar steel alloys are notoriously difficult to activate for electroplating. Furthermore, dissimilar metals are susceptible to galvanic corrosion. Since voids, crevices, cracks and the like entrap electrolytes used in coloring the metals, the requirement for absence of surface imperfections is even more stringent for clips which are color coded in this manner.